When the stimulable phosphor is exposed to radiation such as X-rays, it absorbs and stores a portion of the radiation energy. The stimulable phosphor then emits stimulated emission according to the level of the stored energy when the phosphor is epposed to electromagnetic wave such as visible light or infrared rays (i.e., stimulating light).
A radiation image recording and reproducing method utilizing the stimulable phosphor has been widely employed in practice. The method employs a radiation image storage panel comprising the stimulable phosphor, and comprises the steps of causing the stimulable phosphor of the storage panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with a stimulating light to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric signals giving a visible radiation image. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as stimulable phosphor sheet) has a basic structure comprising a substrate and a stimulable phosphor layer provided thereon.
The phosphor layer is generally formed by coating a dispersion of phosphor particles in a binder solution on the substrate and drying the coated dispersion on the substrate, and therefore comprises a binder and phosphor particles dispersed therein.
It is desired that radiation image storage panels used in these methods have sensitivity as high as possible.
It is known that a radiation image storage panel having on a substrate a stimulable phosphor film prepared by vapor deposition (or vapor-accumulating method) such as vacuum vapor deposition or sputtering gives a reproduced radiation image with high sensitivity as well as high sharpness.
Japanese Patent Provisional Publication No. 62-47600 discloses a method in which a stimulable phosphor film of a radiation image storage panel is formed by electron beam evaporation (which is a kind of vapor deposition method). In the method, an electron beam generated by an electron gun is applied onto a stimulable phosphor or its starting materials (i.e., evaporation source) to heat and vaporize the source, to deposit the vapor to form a phosphor film on the surface of the substrate. Thus formed phosphor film consists essentially of prismatic crystals of the stimulable phosphor. In the phosphor film, there are cracks among the prismatic crystals of the stimulable phosphor. For this reason, the stimulating rays are efficiently applied to the phosphor and the stimulated emission are also efficiently taken out. Hence, a radiation image of high sharpness can be obtained with high sensitivity.
Japanese Patent Publication No. 6-77079 describes a radiation image storage panel in which a stimulable phosphor film is formed by vapor deposition to have a fine block structure. A wide variety of stimulable phosphors are described in the publication as phosphors employable in the storage panel. However, only a thallium activated rubidium bromide phosphor is mentioned in detail in the publication.
Japanese Patent Publication No. 6-100679 describes a radiation image storage panel comprising a phosphor film of thallium activated rubidium halide which is prepared by heating plural evaporation sources with controlling evaporation rates of these evaporation sources,
Japanese Patent Publication No. 7-84588 describes a radiation image storage panel having a stimulable phosphor layer using an alkali metal (Rb or Cs) halide phosphor. In the publication, the phosphor film is produced by coating a phosphor-binder dispersion on a support. The alkali metal halide is defined to contain an activator element of not more than 20 atomic %.
Japanese Patent Publication No. 5-32945 describes an apparatus for a radiation image-reproducing process comprising the steps of exposing a radiation image storage panel to stimulating rays having emitted from a light source (e.g., a fluorescent lamp) through a slit for linearly stimulating the storage panel (i.e., line stimulation), and detecting the stimulated emission having emitted from the storage panel by a line sensor comprising many photoelectric converting elements (i.e., line detection). The line sensor is placed so that it can face the storage panel in the area exposed to the stimulating rays on the exposed side or its back side.